Method for the production of metal powder for powder metallurgical purposes



NIETHOD FOR PRODUCTION OF METAL POWDER FOR POWDER METALLURGICAL PURPOSES Nils Herman Brundin, Hoganas, Sweden, assignor to Hoganas-Billesholms Aktiebolag, Hoganas, Sweden, :1 Swedish joint stock company Application December 27, 1950 Serial No. 203,034

- 12 Claims. 01. 75-413 v No Drawing.

'In the production of mouldings by powder metallurgy a body of desired shape is pressed from metal powder, said body being then sintered to obtain the necessary increased tensile strength.

The grain size of the powder is of decisive importance in the production according to this method. In general terms, a'small grain size improves the mechanical properties, especially the tensile strength of the finished mouldings. However, in practice one cannot operate with too small grain sizes for the following reasons:

(1) A very fine grain powder has a bed flowing capacity, i. e. it forms a coherent mass which only with difliculty can be caused to flow through narrow openings. For this reason the use of automatic presses is rendered difiicult and the powder can only with difliculty or not at all be caused to flow down into the moulding tool.

' (2) At the pressing the fine powder penetrates into the parts of the moulding tool which are movable relatively to each other, and causes an increased wear of the sliding surfaces.

-(3) A moulding produced from fine powder shrinks considerably at the sintering which, due to the irregularity of the shrinking in various directions, causes great variations of the final dimensions of the moulding.

For this reason one must in practice generally use a relatively coarse grain powder (up to about 015mm.) and forego the advantages of the very fine grain powders.

The present invention relates to a method for the production of a metal 'powder which, whenused for powder metallurgical purposes, combines the good compressible properties of the coarse powder with the advantageous influence of the fine grain powder upon the strength of the finished mouldings. In order to attain this purpose, the invention is substantially characterized in that the metal powder used as'starting material is first ground to such a small grain size that the flowing capacity of this powder becomes insufficient for powder metallurgical moulding, whereafter said fine powder is agglomerated to form larger aggregates which are then comminuted to such a grain size that the desired flowing capacity is obtaned. The agglomeration of said too fine powder to form aggregates which are too large can be effected either by a sintering process or by the use of organic binding agents. The first crushing is preferably made to a grain size of 0.05 mm. at the most, while the finally produced powder is preferably given a maximum grain size of about 0.15 mm. If the agglomeration of the fine crushed powder is'efiecte'd by a sintering process and there is a risk of oxidation of the powder during the sintering, said sintering is preferably carried out in a neutral or reducing atmosphere.

The method according to the invention is applicable not only to the production of unalloyed metal powder but it may also with advantage be used in the production of metal powder for alloyed mouldings of powder metal. The method nearest at hand for the production of such mouldings consists in producing an alloy of the desired composition and converting said alloy to a metal powder which is finally pressed and sintered to the desired moulding.

However, this method is not very advantageous in practice because the alloyed powder is generally essentially harder than its components. For this reason the compressibility of the powder is reduced and it will be necessary to use greater moulding pressure to produce a moulding having a certain density, which involves an increased wear of the moulding tools and increased costs.

It is also possible to mix the alloying components in the desired proportions in powder form, the alloy being formed by diffusion during the sintering. This alloying takes place in the solid phase if the sintering temperature lies below the melting points of the components, and

partly in the solid phase, partly in the liquid phase, if

the sintering temperature lies above the melting point of one of the components. A good compressibility of the powder mixture is obtained by using this method. However, the diffusion rate is in many cases so low that acomplete alloying is not obtained within reasonable sintering periods.

A much better result is obtained by using the method according to the invention, comprising first producing a very fine grain metal powder containing the alloying components and then binding, e. g. sintering-this powder cautiously to a grindable body which is crushed to a powder having a greater grain size than that of the original powder. The starting material may be produced in many different ways. For example, finegrain powder of the alloying components maybe mixed in suitable proportions. One or more of the alloying components may also be chemicaly or electrolytically precipitated on a pulverulent alloying component. Various combinations of'these methods are, of course, also possible. For example, two or more of the alloying components may be 'added in pulverulent state while components are precipitated on the powder grains.

The following examples for applying the invention in practice may be stated.

Example 1 Rods suitable to be subjected to tensile strength tests were manufactured by pressing at a pressure of 5 tons per cm. from the following materials:

(1) Iron powder with a grain size of maximum 0.15 mm.

(2) Iron powder with a grain (3) Iron powder produced according to the invention in the following manner: Material according to (2) was heated under reducing conditions at 950 C. for a period of one hour. The body obtained was then ground so that all powder passed through a screen having a free mesh width of 0.15 mm. and thus obtained about the same grain size as the powder according to (1).

size of maximum 0.05

After pressing, the rods were sintered for one hour 1 at 1100 C. in hydrogen gas atmosphere and their tensile strength was then tested. The following results were obtained. Y 1

Test number:

strength. However, as is clear from the viewpoints cited in the introductory part of this specification the powder 2 cannot be used for technicalypurposes while the, pow- Patented Oct. 21, 1958:

Tensile strength kg./mm.

ders 1 and 3 are completely equivalent from this viewpoint.

Example 2 A pulverulent mixture was produced from 7.5% copper and 92.5% ironin the following four different ways. (1) Iron powder and copper powder, both having a grain size of 0.15 mm. at the most, were mixed for one hour in a rotating drum.

(2) On iron powder having a grain size of 0.15 mm.

at the most copper was precipitated chemically in the form of a layer on the grains, whereafter the powder was dried and heated at 700 C. in hydrogen atmosphere to removeany hydroxide formed. Only a slight sintering to smallaggregates was obtained, which were comminuted by a gentle crushing whereby the powder obtained sub- L stantially the same grain size as the starting, material.

(3) Iron powder andcopper powder having a grain size of 0.05 mm. were mixed for one hour ina rotating drum. Thereafter the mixture was heated for one hour at 800 C. toasolid sintered cake which was then groundto a powder having a grain size of 015mm. at the most. (4) On iron powder having a grain size of 0.05 mm. at the most, copper was precipitated chemically where-. after the powder was siutered and ground inthe same manner as according to (3).

The difierent iron powders obtained according to the above methods thus show the same grain size (less-than Test number: Tensile strength leg/mm.

The rods 3 and 4 manufactured from the powders according to the invention are thus considerably superior to the two other rods. This is a consequence of the fact that due to the small grain size of the powder there are more points of contact between the iron and the copper in the first-mentioned rods, and therefore the molecules of the two substances have to travel a shorter distance at the alloying during the preliminary as well as during the final sintering resulting in that the alloying takes place at a greater rate. Thus, the time required for a complete alloying is considerably reduced and at the same time the structure becomes much more fine-granular than would otherwise be the case, and the mechanical properties are improved.

To the metal powder, the grain size of which is so small that it has a flowing capacity which is insufficient for powder metallurgical purposes, there may according to one embodiment of the invention be added small quantities of an organic substance which by a treatment suited to the character of the substance, agglomerates the grains to larger aggregates which are comminuted to the desired grain size which is larger than that of the starting material, by screening or cautious crushing, for instance by pressing through a screen.

In this case, as in the sintering process described above, it is suitable to grind the original powder to a grain size of about 0.05 at the most, and, after its aggregation, to comminute the aggregate again to a grain size of up to 0.15 mm.

The obtained powder which possesses the desirable properties of a course powder, especially its flowing capacity, is pressed and thereafter sintered in the manner usual in powder metallurgy.

on used. as lubricants for the moulding proper in the i The organic substance may consist of a binding agent. which is soluble in an organic solvent, e. g. alcohol, ether,

benzol, etc. In this case thefbinding agent is preferably added in solution, inorder to, be uniformly distributed in the powder in a subsequent mixing process. After the admixture the solvent is evaporated by being heated to a temperature suited to thecharacter of the solvent, preferably under such conditions that the solvent can be recovered by condensatoinand be used again.

Cellulose lacquers, e. g. nitro cellulose, shellac, colophony, phenol resin and so on may be mentioned as examples of binding agents of this type which have proved fit for use. g

It is, of course, also possible to use a binding agent which is soluble in water. Yet, such a binding agent easily causes the-iron powder to be attacked by rust which is not desirable.

Another type of substances which may be used according to the invention are those which becomesoft or melt upon heating and thereby bind the grainstogether, e.g. paraflin or waxy substances. These substances may be admixed inthe form ofta fine powder whereafter the mixture is heated to the softeningor melting temperature of the admixed substances. It is, of course, also possible to admix these substances in the formof a solution.

A class of substances which is particularly advantageous to use according to the invention, consists of the substances ofthe type: stearic acid, zinc stcarate and so powder metallurgy, these substances contributing to the agglomeration of thefine powder to improve (its flowing capacity andalso having a lubricating ettect at the moulding.

According to the invention it is, of course, possibleto add two or more different binding agents, oneof which at least shouldpossess lubricating properties. In this case it may also be suitable to add one agent in the form of a powder and another agent in the form of a solution.

The heat treatment following the mixing should be carried out at a temperature which is lower than that at which the metal powder begins to oxidize, if a protecting atmosphere is not used at the heating.

The abovementioned possibility of using afine powder for powder metallurgical purposes is of particular advantage when it is the question of producing alloys in the powder metallurgical way by pressing andsintering of mixtures of different metal powders. Of course, one can also use as starting material a powder the individual grains of which consist of two or more metals either in the form of aggregates of mono-metallic powder grains or a core of. ametaland layers of one or more other metals precipitated electrolytically or chemically on said core. The possibility of using very fine grain powders according to the invention considerably reducesthe time required for the alloying at the sintering temperature.

What I claim is:

1. Method for the production of metal powder for powder metallurgy which consists essentially of agglomerating a metal powder having a particle size not greater than .05 mm. to the production of aggregates having a size substantially greater than 0.15 mm. and disintegrating said aggregates to a particle size greater than .05 mm. but not substantially greater than 0.15 mm.

2. Method as defined in claim 1 in which the agglomerationis effected by sintering.

3. Method as defined in claim 1 in which the metal powder having a particle size not greater than. .05 mm. is a mono metallic powder.

4. Method as defined in claim 1 in which the metal powder having a particle sizenot greater than .05 mm. consists of at least two different metals.

5. Method as defined in claim 1 in which the metal powderhaving a particle size not greater than .05 mm. is a mixture of at least two mono metallic metal powders and in which the agglomeration is effected by sintering without substantial alloying of said difierent metals.

6. Method as defined in claim 1 in which the metal.

powder having a particle size not greater than .05 mm. comprises particles of one metal having a coating of another metal precipitated thereon.

7. Method as defined in claim 1 in which the metal powder is agglomerated by means of an organic material having binding properties.

8. Method as defined in claim 1 in which the metal particles are agglomerated by means of a fusible organic material.

9. Method as defined in claim 1 in which the metal particles are agglomerated by means of an organic material dissolved in a volatile solvent.

10. Method as defined in claim 1 in which the metal particles are agglomerated by means of an organic ma terial having lubricating properties.

11. Method as defined in claim 1 in which the metal particles are agglomerated by means of a mixture of an organic substance having binding properties and a lubricant.

12. A method of producing strongly coherent sintered bodies from metal powder which consists essentially of agglomerating a metal powder having a particle size not greater than 0.05 mm. to produce aggregates having a size substantially greater than 0.15 mm., disintegrating said aggregates to produce a powder having a maximum grain size substantially greater than 0.05 mm. but not greater than O.l5 mm., pressing the last named powder to a coherent body and heating said body to sintering temperature.

References Cited in the file of this patent UNITED STATES PATENTS 1,697,402 Nutter et a1. Jan. 1, 1929 2,096,924 Schwarzkopf Oct. 26, 1937 2,179,960 Schwarzkopf Nov. 14, 1939 2,200,369 Klinker May 14, 1940 2,306,665 Schwarzkopf Dec. 29, 1942 2,541,671 Segura et al. Feb. 13, 1951 

1. METHOD FOR THE PRODUCTION OF METAL POWDER FOR POWDER METALLURGY WHICH CONSISTS ESSENTIALLY OF AGGLOMERATING A METAL POWDER HAVING A PARTICLE SIZE NOT GREATER THAN .05 MM. TO THE PRODUCTION OF AGGREGATES HAVING A SIZE SUBSTANTIALLY GREATER THAN 0.15 MM. AND DISINTEGRATING SAID AGGREGATES TO A PARTICLE SIZE GREATER THAN .05 MM. BUT NOT SUBSTANTIALLY GREATER THAN 0.15 MM. 